1. Field of the Invention
The present invention relates mainly to a full-range speaker used widely in stereos, multi-channel sound reproduction devices, radios, and televisions.
2. Description of Related Art
In recent years, many speakers have been installed in homes as multi-channel home theater reproduction devices or the like become widely available. For this reason, further miniaturization and cost reduction of the speakers have been required. Therefore, instead of using the speakers in a multi-way configuration, it is desired that a single speaker be capable of reproducing low to high frequencies with high sound quality.
In general, however, it is difficult for a single speaker to reproduce low to high frequencies with excellent sound quality, and in particular to reproduce high frequencies with an excellent response. This is because the diameter of a speaker cannot be too small in order to reproduce the low frequencies to some extent, so that a reproducible frequency range at high frequencies is narrowed, and the directivity at high frequencies is particularly degraded. The difficulty in reproducing high frequencies with a single speaker with an excellent response is well known.
To solve this problem, a speaker with an improved high frequency response, such as a speaker described in FIG. 5.2, page 145 of “Speaker System Vol. 1” (Takeo Yamamoto ed., First published on Jul. 15, 1977, Radio Technology Co. Ltd.), has been proposed, and several speakers of this kind have come onto the market. FIG. 8 is a configuration diagram of a conventional speaker with an improved high frequency response described in the above-mentioned document. Hereinafter, a description will be given with reference to FIG. 8.
In FIG. 8, a field magnet 31, a damper 34, and a speaker edge 37 are attached to a frame 32, and a voice coil 33 is supported by the damper 34. A diaphragm 35 has a cone shape, and its inner periphery is coupled to the voice coil 33 and its outer periphery is supported by the speaker edge 37. A dust cap 36 is attached to an inner peripheral part of the diaphragm 35. A corrugation 35a is provided at the intermediate position of the diaphragm 35.
With this configuration, though the entire diaphragm 35 vibrates at low frequencies, the corrugation 35a functions as a mechanical filter at high frequencies. Therefore, vibrations at high frequencies are not likely to be transmitted to an outer peripheral part of the diaphragm 35 outside the corrugation 35a. As a result, only an inner peripheral part of the diaphragm 35 inside the corrugation 35a mainly vibrates, thereby improving the high frequency response.
However, in the above-described conventional speaker, the mechanical compliance of the corrugation 35a has to be increased significantly in order to suppress the vibration transmission at high frequencies to the outer peripheral part of the diaphragm 35 outside the corrugation 35a. Consequently, a mid-high frequency response is remarkably degraded because a vibration mode in an outer peripheral part of the diaphragm 35 is disturbed or a resonance occurs.
In order to prevent the degradation of the mid-high frequency response, the corrugation 35a must have a very small mechanical compliance. Thus, the vibration transmission at high frequencies to the outer peripheral part of the diaphragm 35 cannot be suppressed only with the corrugation 35a, so that the high frequency response cannot be improved.
Recently, with the progress of a diaphragm material or the like, the difficulty in expanding the reproducible frequency range at high frequencies has been gradually diminished as far as in a direction of the axis of the speaker is concerned. However, the directivity in a direction away from the axis of the speaker cannot be improved. This is because, due to the vibration transmission at high frequencies to the outer peripheral part of the diaphragm 35, high frequencies also are radiated from the outer peripheral part of the diaphragm 35, and thus the effective vibration area is not reduced.
On the other hand, there has been known a method in which a sub-cone for reproducing high frequencies is added instead of providing a corrugation on a diaphragm. However, high frequency sounds radiated from the diaphragm and high frequency sounds radiated from the sub-cone interfere with each other to cause the degradation of sound quality. Accordingly, it is desirable that the high frequency response can be improved with a single diaphragm.